[R] data.table: “group counter” with NAs

[Marine Regis]

Hello,

I would like to add a counter column in a data frame based on a set of identical rows. To do this, I tested:

DF = data.table(x=c("a","a","a","b","c","d","e","f","f"), y=c(1,3,2,8,8,4,NA,NA,NA))

DF[ , Index := .GRP, by = c("y") ]

DF



However, the rows with NAs are considered to be identical.

So, how can I obtain:

   x  y Index

1: a  1     1

2: a  3     2

3: a  2     3

4: b  8     4

5: c  8     4

6: d  4     5

7: e NA     6

8: f NA     7

9: f NA     8



Instead of:

x  y Index

1: a  1     1

2: a  3     2

3: a  2     3

4: b  8     4

5: c  8     4

6: d  4     5

7: e NA     6

8: f NA     6

9: f NA     6





Thank you very much for your time.

Have a nice day

Marine


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